High intensity discharge (HID) lamps are typically used where large areas require illumination, such as in factories, parking lots and sports arenas. In some applications, such as illuminating a sports arena during a sporting event, after a momentary power failure that terminates illumination by the lamps, it is naturally desired that the lamps rapidly restart to allow the sporting event to continue. However, a hot HID lamp typically requires a high current at an elevated voltage to cause the lamp to drop in voltage to where its power supply, or ballast, circuit can sustain lamp operation.
The above cross-referenced application entitled "System for Starting a High Intensity Discharge Lamp," Ser. No. 08/306,342, is directed to an improved hot restrike circuit for HID lamps that performs well over a long life. With reference to terminology employed herein, the hot restrike circuit of the co-pending application includes a starting circuit in which a starting capacitor is charged through a resistor, and discharged through the primary winding of a pulse transformer and a current switch when it becomes conductive (i.e. turns on). A secondary winding of the pulse transformer provides a high voltage, hot restrike pulse that is applied across the lamp to initiate lamp starting. Multiple high voltage, hot restrike pulses per half cycle of lamp-driving voltage can be provided to assure the high current at an elevated voltage needed to initiate lamp turn-on.
The current switch disclosed in the co-pending application has a control electrode that is controlled by a starting aid, which may be conventional per se. The specific current switch disclosed in the co-pending application is a three-electrode spark gap device that has a main spark gap formed between a pair of main electrodes, and a triggering spark gap formed between one of the main electrodes and a trigger (or control) electrode.
The starting aid, or "trigger" circuit as used herein, which provides the control signal for the current switch of the starting circuit, may typically include a trigger capacitor that is charged through a resistor. The trigger capacitor is then discharged through the primary winding of a pulse transformer, which has a secondary winding that generates a trigger pulse as the control signal to trigger into conduction the current switch of the starting circuit.
The use of a conventional starting aid in the hot restrike circuit of the co-pending application has enabled rapid restarting of an HID lamp of the metal halide variety. However, the present inventor has discovered that even further improvements in hot restrike capability can be achieved using the principles of the present invention. One improvement is to regulate the intensity of hot restrike pulses so that they are consistently at a high level. As such, the hot restrike pulses are more effective at delivering high current to the lamp. A further improvement is to increase the reliability of obtaining multiple hot restrike pulses during a half cycle of lamp-driving voltage, which also contributes to the effectiveness of the hot restrike pulses.